Most power transmission parts (for example, gears, bearings, CVT sheaves, shafts, and the like) used for automobiles, construction machines, farm machines, electricity generating-wind turbines, other industrial machines and the like are used after being subjected to a surface hardening treatment to improve, for example, fatigue characteristics and abrasion resistance of the parts. Among a plurality of known surface hardening treatments, a carburizing treatment is superior to other surface hardening treatments in terms of the surface hardness, depth of hardened layer, productivity, and the like, so the carburizing treatment is applied to numerous types of the parts. For example, in a manufacturing method of the gears and bearing parts, by using medium carbon alloy steel such as SCM 420, SCR 420, SNCM 220 in general, and the like specified by JIS, a mechanical processing is performed to obtain a predetermined shape through a hot forging, a cold forging, cutting, or through a combination thereof, and then the carburizing treatment or a carbonitriding treatment is performed. The fatigue fracture of the gears is classified roughly into bending fatigue (dedendum fatigue) and tooth surface fatigue (pitting or the like). In order for the gear parts to obtain durability, both the above types of fatigue strength need to be improved. Since the gears which are produced by the carburizing treatment have very high hardness of hardening layer, there is the feature of having the performance excellent in both the bending fatigue strength and the fatigue strength.
However, the carburizing treatment is batch processing in the gas atmosphere. For example, the carburizing treatment requires heating and holding around 930° C. for several hours or more, so that significant occupancy expense, treatment energy, and cost are needed. The carburizing treatment emits a large volume of CO2, so that there is a problem in terms of the environment. Since the carburizing treatment is the batch processing, the carburizing treatment has the problems which are that the dispersion of part accuracy becomes possibly large because of the heat treatment deformation caused by the difference of the loading position of the parts at the carburizing treatment and that the accuracy control of the parts is difficult. In order to solve the problems concerned with the heat treatment deformation, huge effort has been made in regard to materials and operations, and then the improvement effect to a certain extent has been obtained. However, a radical method of settlement is still not found out, and it is said that the improvement effect does not reach enough levels.
In order to solve the problems, the research on application of induction hardening (electromagnetic induction hardening) treatment to substitute the carburizing treatment has been made. Since the induction hardening treatment can reduce considerably the energy and the time for the treatment compared with the carburizing treatment, the induction hardening treatment has the advantages of the productivity and the cost reduction. Furthermore, the induction hardening treatment does not emit a large volume of CO2 and quenching oil to environment, so that there is the advantage for the environment. In addition, since the area which is affected by the influence of the heat treatment is limited to the surface unlike the carburizing treatment, the heat treatment deformation by the induction hardening treatment is essentially small. Moreover, there are the advantages which are that consecutive processing becomes possible because processing time is short and that the accuracy control of the parts becomes easy because the dispersion of the heat treatment deformation is small.
On the other hand, although there are the above mentioned advantages, the induction hardening treatment has not become common as the substitution of the carburizing treatment. The prime reasons thereof are because coexistence between the securement of tooth surface fatigue strength (pitching strength and the like) of the parts and workability (machinability or cold forgeability) during production of the parts is very difficult. Not only the gears but also CVT sheaves and bearings need to improve surface fatigue such as tooth surface fatigue and rolling fatigue. It is reported that hardness at 300° C. (or hardness after tempering at 300° C., hereinafter referred to as 300° C. tempered hardness) correlates strongly with the surface fatigue strength since surface temperature of the contact surface of the parts rises up to about 300° C. while using the parts. The 300° C. tempered hardness of martensite structure obtained by the carburizing treatment or the induction hardening treatment improves with an increase in carbon content in surface layer. Although the 300° C. tempered hardness is affected by addition of alloying elements, the influence of the carbon content is greater. An improvement effect of the 300° C. tempered hardness by the addition of the alloying elements increases with the increase in the carbon content. Therefore, in order to obtain the surface fatigue strength equivalent to carburized parts, it is necessary that the carbon content (about 0.80%) is equivalent to the carbon content in the surface layer of the carburized parts. However, the increase in the carbon content of the parts results in the increase in hardness of base steel, so that the workability (the machinability or the cold forgeability) of the parts deteriorates remarkably, which is not suitable for industrial production. It is indispensable to coexist between high carbon content of the base steel and securement of the workability.
For example, Patent Documents 1 to 6 suggest the technique of producing parts by performing the induction hardening to medium carbon steels (C: to 0.65%). However, since the carbon content is considerably less than that of the surface layer of the carburized parts, the workability does not deteriorate so much, but the tooth surface fatigue strength decreases compared with the carburized parts. For this reason, the technique cannot be substituted for the carburizing. For example, Patent Documents 7 to 13 suggest the technique of obtaining the parts in which the tooth surface fatigue strength is improved by performing the induction hardening to comparative high carbon steels (C: to 0.75%). However, since the carbon content is still less than that of the surface layer of the carburized parts, the tooth surface fatigue strength which is equal to that of the carburized parts is not obtained. Moreover, the workability decreases notably with the increase in the carbon content in the steels. However, since the improvement technique for this is insufficient, both the tooth surface fatigue strength and the workability are insufficient after all, so that the technique cannot be substituted for the carburizing.
For example, Patent Documents 14 to 17 suggest the technique which is to improve the workability and the like by providing suitable rolling conditions, forging conditions, and cooling conditions to comparative high carbon steels (C: to 0.75%). However, as described above, since the carbon content is still less than that of the surface layer of the carburized parts, the tooth surface fatigue strength which is equal to that of the carburized parts is not obtained, so that the technique cannot be substituted for the carburizing.
For example, Patent Documents 18 to 23 suggest the technique in which a heat treatment is performed if necessary and then the induction hardening is performed to the steels which have the high carbon composition which is equal to that of the surface layer of the carburized parts. By the technique, a hardening layer with the structure in which alloy carbides are dispersed in the martensite structure is formed, so that the parts which have the high tooth surface fatigue strength are obtained. However, in the technique, alloy addition such as Cr, V, Ti, Nb, and the like is large in order to disperse the alloy carbides. Thus, although the tooth surface fatigue strength which is greater than that of the carburized parts is obtained, the workability decreases notably by both the increase of the carbon content and the increase of the alloy addition. Therefore, except for application on some special parts, since the application and the practical realization for mass products are difficult in terms of the cost, the productivity, and the like, it cannot be said that the technique is practical to substitute for the carburizing.
For example, Patent Documents 24 to 26 suggest the technique in which the heat treatment is performed if necessary and then the induction hardening is performed to the steels which have the high carbon composition which is equal to that of the surface layer of the carburized parts, in order to obtain the parts in which the tooth surface fatigue strength is improved. However, since the improvement for the workability is insufficient, the technique also cannot be substituted for the carburizing.
For example, Patent Document 27 suggests the technique which is to improve the machinability by precipitating graphites to a certain extent by using the high carbon steels (C: 0.80 to 1.50%). Although the example of application to the induction hardening steel part is also shown in the patent documents 27, in the base steel in which a lot of the graphites are dispersed, it is difficult to solute the graphites as solid solution in matrix, and there is a problem such that voids are formed at the position where the graphites existed. For this reason, in the method, various characteristics for the power transmission parts which require reliability deteriorate. In order to perform solution of the graphites or dissolution of the voids, the induction hardening should be performed by the special conditions which are at a high temperature and for a long time. For this reason, problems which are that the control of depth of the hardening layer is impossible or the productivity deteriorates occur. In this case, the above mentioned advantageous feature of the induction hardening is not obtained at all. Therefore, it cannot be said that the technique of dispersing a lot of graphites is practical to apply to the induction hardening treatment of the power transmission parts.